The present invention relates to a method of authenticating a picture embedding various kinds of data such as a logo mark, a sales date and a buyer or a user as information for authentication.
A technology of embedding various kinds of hidden data such as a logo mark, a sales date and a buyer as information for authentication is in general known as a digital-watermark technology which is described in a technical article entitled xe2x80x9cTechniques for Data Hidingxe2x80x9d, IBM Systems Journal, Vol. 35, 1996, pages 313 to 336.
According to the digital watermark technology, if a seller of a picture produced as a digital literary work embeds data showing a buyer of the picture in the original picture as information for authentication and sells the picture to the buyer as a digital-watermark picture, information corresponding to data for authentication embedded in a picture at a sales time can be extracted from the picture in case there is a doubt that the picture has been manipulated by misconduct. The extracted information can be used to form a judgment as to whether the picture has been indeed manipulated by misconduct. Further, if a result of the judgment indicates that the picture has been indeed manipulated by misconduct, the extracted information can be used for identifying the legal buyer of the picture.
It should be noted that the digital-watermark technology includes a technique that allows information for authentication to be embedded more than once to make the digital-watermark picture proof against partial extraction of the information from the picture.
By the way, a person can conceal misconduct by using the two following conceivable methods:
(1) Embedded information for authentication is removed from a digital-watermark picture.
(2) In order to prevent embedded information for authentication from being extracted, the information is falsified through typically picture transformation.
In the conventional digital-watermark technology, a variety of techniques for preventing the method (1) described above have been proposed but, as a technique for preventing the above method (2), only the technique of embedding information for authentication in a picture more than once is available. The only falsification that can be prevented by the technique of embedding information for authentication in a picture more than once is partial extraction of a digital-watermark picture.
While falsification implemented by geometrical coordinate transformation such as enlargement/shrinkage or rotation results in picture degradation, such falsification prevents the conventional digital-watermark technology from extracting information corresponding to data for authentication with a high degree of precision from a degraded picture since the conventional digital-watermark technology does not take picture degradation into account.
It is thus an object of the present invention to provide a capability of extracting information corresponding to data for authentication with a high degree of precision from a picture even if the picture has been degraded by falsification.
In order to achieve the object described above, the present invention provides a first method of authenticating a digital-watermark picture including the steps of: carrying out an embedding process to create a digital-watermark picture which is a picture embedding information for authentication into an original picture; carrying out an extraction process to extract information corresponding to the information for authentication from the picture being inspected; and carrying out an authentication process to authenticate legitimacy of the picture being inspected based on the information extracted in the extraction process, wherein the extraction process includes the steps of: carrying out a geometric transformation on the picture being inspected to transform the size of the picture being inspected into a reduced scale of the original picture; creating a plurality of degraded original pictures with different types each obtained as a result of degradation of the original picture; creating a plurality of differential pictures each obtained as a result of finding differences between the picture being inspected with a transformed size and one of the degraded original pictures with different types; and extracting information corresponding to the information for authentication from each of the differential pictures.
In addition, in order to achieve the object described above, the present invention provides a second method of authenticating a digital-watermark picture including the steps of: carrying out an embedding process to create a digital-watermark picture which is a picture embedding information for authentication in an original picture; carrying out an extraction process to extract information corresponding to the information for authentication from the picture being inspected; and carrying out an authentication process to authenticate legitimacy of the picture being inspected based on the information extracted in the extraction process, wherein the extraction process includes the steps of: carrying out a geometric transformation on the picture being inspected to transform the size of the picture being inspected into a reduced scale of the original picture; creating a plurality of degraded original pictures with different types each obtained as a result of degradation of the original picture; creating a plurality of differential pictures each obtained as a result of finding differences between the picture being inspected with a transformed size and one of the degraded original pictures with different types; and displaying the differential pictures and extracting information corresponding to the information for authentication from one of the displayed differential pictures specified externally.
Furthermore, in order to achieve the object described above, the present invention provides a third method of authenticating a digital-watermark picture including the steps of: carrying out an embedding process to create a digital-watermark picture which is a picture embedding information for authentication in an original picture; carrying out an extraction process to extract information corresponding to the information for authentication from the picture being inspected; and carrying out an authentication process to authenticate legitimacy of the picture being inspected based on the information extracted in the extraction process, wherein the extraction process includes the steps of: carrying out a geometric transformation on the picture being inspected to transform the size of the picture being inspected into a reduced scale of the original picture; creating a plurality of degraded original pictures with different types each obtained as a result of degradation of the original picture; creating a plurality of differential pictures each obtained as a result of finding differences between the picture being inspected with a transformed size and one of the degraded original pictures with different types; computing a statistic of errors for each of the differential pictures; and extracting information corresponding to the information for authentication from one of the differential pictures with a smallest computed statistic.
To be more specific, according to a concept provided by the present invention, the extraction process includes the steps of: carrying out a geometric transformation on the picture being inspected to transform the size of the picture being inspected into a reduced scale of the original picture; inferring the degree of picture degradation of the picture being inspected with a transformed size relative to the original picture; creating a degraded original picture obtained as a result of degradation of the original picture by the degree of picture degradation inferred; creating a differential picture obtained as a result of finding differences between the picture being inspected with a transformed size and the degraded original picture; and extracting information corresponding to the information for authentication from the differential picture.
Given that inference of the degree of picture degradation is the real problem to be solved, in order to allow the degree of picture degradation to be inferred, a plurality of degraded original pictures with different types are created in advance.
In either of the methods given by the present invention, a plurality of degraded original pictures with different types each obtained as a result of degradation of the original picture can be obtained in the extraction process which includes the steps of: computing geometric-transformation coefficients required for geometric transformation of the original picture into a temporary picture with a size of a reduced scale of the picture being inspected; and using a plurality of interpolation techniques of different types prepared in advance to carry out geometric transformation on the original picture to transform the original picture into temporary pictures each with a size of a reduced scale of the picture being inspected on the basis of the geometric-transformation coefficients to transform the size of each of the temporary pictures back into an original size on the basis of the geometric-transformation coefficients.
In addition, in the extraction process of either of the methods described above, it is possible to carry out a geometric transformation on the picture being inspected to transform the size of the picture being inspected into a reduced scale of the original picture on the basis of geometric-transformation coefficients which are required for the geometric transformation of the picture being inspected and computed from a plurality of tie points determining tie positions between the picture being inspected and the original picture.